**Differential pressure transmitters used for liquid level applications measure ****hydrostatic pressure head. Liquid level and specific gravity of a liquid are ****factors in determining pressure head. This pressure is equal to the liquid ****height above the tap multiplied by the specific gravity of the liquid. Pressure ****head is independent of volume or vessel shape.**

**Open Vessels :- A pressure transmitter mounted near a tank bottom measures the pressure of ****the liquid above.****Make a connection to the high pressure side of the transmitter, and vent the ****low pressure side to the atmosphere. Pressure head equals the liquid’s****specific gravity multiplied by the liquid height above the tap. ****Zero range suppression is required if the transmitter lies below the zero point ****of the desired level range. Figure shows a liquid level measurement ****example.**

**Closed Vessels :- Pressure above a liquid affects the pressure measured at the bottom of a ****closed vessel. The liquid specific gravity multiplied by the liquid height plus ****the vessel pressure equals the pressure at the bottom of the vessel. ****To measure true level, the vessel pressure must be subtracted from the ****vessel bottom pressure. To do this, make a pressure tap at the top of the ****vessel and connect this to the low side of the transmitter. Vessel pressure is ****then equally applied to both the high and low sides of the transmitter. The ****resulting differential pressure is proportional to liquid height multiplied by the ****liquid specific gravity.**

**Dry Leg Condition**

**Low-side transmitter piping will remain empty if gas above the liquid does not ****condense. This is a dry leg condition. Range determination calculations are ****the same as those described for bottom-mounted transmitters in open ****vessels, as shown in Figure**